System and method for electrochemical cell system and leak detection and indication

ABSTRACT

An electrochemical cell system is provided having: at least one electrochemical cell stack, each stack having at least one reactant fluid inlet; a pressure transmitter arranged in the at least one reactant fluid inlet of each stack; and a control unit for regulating the electrochemical cell system, the control unit receiving at least one signal value from the pressure transmitter indicative of the reactant fluid pressure. The control unit may compare the at least one signal value with a stored values and generate a leak indication based on the rate of pressure decay within the electrochemical cell system. A method of detecting and indicating a leak is also disclosed.

PRIORITY

This application claims the benefit of U.S. Provisional PatentApplication No. 60/862,559, filed Oct. 23, 2006.

FIELD

This invention relates to an electrochemical cell system, and moreparticularly relates to an apparatus and a method of detecting andindicating leaks of the cell stack.

BACKGROUND

The following paragraphs are not an admission that anything discussed inthem is prior art or part of the knowledge of persons skilled in theart.

Electrochemical cells are energy conversion devices and usually are usedto collectively indicate fuel cells and electrolyzer cells.

Fuel cells have been proposed as a clean, efficient and environmentallyfriendly source of power that can be utilized for various applications.A conventional proton exchange membrane (PEM) fuel cell is typicallycomprised of an anode, a cathode, and a selective electrolytic membranedisposed between the two electrodes. A fuel cell generates electricityby bringing a fuel gas (typically hydrogen) and an oxidant gas(typically oxygen) respectively to the anode and the cathode. Inreaction, a fuel such as hydrogen is oxidized at the anode to formcations (protons) and electrons by the reaction H₂=2H⁺+2e−. The protonexchange membrane facilitates the migration of protons from the anode tothe cathode while preventing the electrons from passing through themembrane. As a result, the electrons are forced to flow through anexternal circuit thus providing an electrical current. At the cathode,oxygen reacts with electrons returned from the electrical circuit andwith the protons that have crossed the membrane to form liquid water asthe reaction by-product following ½O₂+2H⁺+2e−=H₂O.

On the other hand, an electrolyzer uses electricity to electrolyze waterto generate oxygen from its anode and hydrogen from its cathode. Similarto a fuel cell, a typical solid polymer water electrolyzer (SPWE) orproton exchange membrane (PEM) electrolyzer is also comprised of ananode, a cathode and a proton exchange membrane disposed between the twoelectrodes. Water is introduced to, for example, the anode of theelectrolyzer, which is connected to the positive pole of a suitabledirect current voltage. Oxygen is produced at the anode by the reactionH₂O=½O₂+2H⁺+2e−. The protons then migrate from the anode to the cathodethrough the membrane. On the cathode, which is connected to the negativepole of the direct current voltage, the protons conducted through themembrane are reduced to hydrogen following 2H⁺+2e−=H₂.

A typical electrochemical cell employing PEM comprises an anode flowfield plate, a cathode flow field plate, and a membrane electrodeassembly (MEA) disposed between the anode and cathode flow field plates.Each reactant flow field plate has an inlet region, an outlet region,and open-faced channels to fluidly connect the inlet to the outlet, andprovide a way for distributing the reactant gases to the outer surfacesof the MEA. The MEA comprises a PEM disposed between an anode catalystlayer and a cathode catalyst layer. A first gas diffusion layer (GDL) isdisposed between the anode catalyst layer and the anode flow fieldplate, and a second GDL is disposed between the cathode catalyst layerand the cathode flow field plate. The GDLs facilitate the diffusion ofthe reactant gas, either the fuel or oxidant, to the catalyst surfacesof the MEA. Furthermore, the GDLs enhance the electrical conductivitybetween each of the anode and cathode flow field plates and theelectrodes.

In practice, fuel cells are not operated as single units. Rather fuelcells are connected in series, stacked one on top of the other, orplaced side-by-side, to form what is usually referred to as a fuel cellstack. The fuel, oxidant and coolant are supplied through respectivedelivery subsystems to the fuel cell stack. Also within the stack arecurrent collectors, cell-to-cell seals and insulation, with requiredpiping and instrumentation provided externally to the fuel cell stack.Fuel cross-over from the anode side of the fuel cell to the cathodeside, or from the cathode side to the anode side, may occur when theseals between cell components are inadequate or when the membrane of theMEA is ruptured. This typically develops as the stack ages. There isgenerally a small cross-over of hydrogen or oxygen occurring naturallyacross the MEA (driven by concentration gradients). The net diffusiondue to the concentration gradient will be from either anode to cathodeor cathode to anode depending on which electrode is at the higherpressure. The presence of fuel on the oxidant side of the fuel cell, oroxygen on the fuel side of the fuel cell, is highly undesirable becauseof the direct combustion reaction that may take place. The heatgenerated during this reaction may cause damage to the membrane and/orother parts of the fuel cell. Un-combusted fuel will follow the cathodeexhaust stream and mix with the oxygen therein. This is undesirablebecause of the risk for such a mixture to ignite (4 percent of hydrogenper volume in air is hydrogen's lower flammability limit). In addition,the hydrogen/fuel side of the stack may develop external leaks overtime. These need to be avoided or designed into the system so thatflammable mixtures are not created in the fuel cell environment.Furthermore, the fuel cell can leak internally due to faulty seals orseparator plates, e.g., leakage from reactant fluid to coolant or viceversa.

Fuel cell stacks have been used as power sources in variousapplications, such as fuel cell powered electric vehicles, residentialpower generator, auxiliary power unit, uninterrupted power source, etc.For fuel cell stacks to be used in power generation applications, manyperipheral devices, conditioning devices are needed since fuel cellstacks rely on peripheral preconditioning devices for optimum or evenproper operation. Extensive piping and plumbing work is also requiredfor connection between such devices.

For example, in the situation where the fuel gas of the fuel cell stackis not pure hydrogen, but rather hydrogen containing material, e.g.,natural gas, a reformer is usually required in the fuel deliverysubsystem for reforming the hydrogen containing material to provide purehydrogen to the fuel cell stack. Moreover, in the situation where theelectrolyte of the fuel cell is a proton exchange membrane, since mostof the membranes currently available requires a wet surface tofacilitate the conduction of protons from the anode to the cathode, andotherwise to maintain the membranes electrically conductive, ahumidifier is usually required to humidify the fuel or oxidant gasbefore it comes into the fuel cell stack. In addition, most conventionalfuel cell systems utilize several heat exchangers in gas and coolantdelivery subsystems to dissipate the heat generated in the fuel cellreaction, provide coolant to the fuel cell stack, and heat or cool theprocess gases. In some applications, the process gases or coolant mayneed to be pressurized before entering the fuel cell stack, and,therefore, compressors and pumps may be added to the deliverysubsystems.

These peripheral devices as well as the fuel cell stacks are oftenpackaged together as a power module (fuel cell power module, FCPM),which will often be disposed in a confined environment, where space islimited, such as vehicular applications or other portable applications.Control and regulation of the FCPM is typically performed by anelectronic control unit (ECU), which forms an integral part of the FCPM.

Therefore, there is a need for an electrochemical cell system thatprovides an indication of leaks in the cell stack or out from the cellstack to an operator of the system.

SUMMARY

The following introduction is intended to introduce the reader to thisspecification but not to define any invention. One or more inventionsmay reside in a combination or sub-combination of the apparatus elementsor method steps described below or in other parts of this document. Theinventor does not waive or disclaim his rights to any invention orinventions disclosed in this specification merely by not describing suchother invention or inventions in the claims.

In accordance with an aspect of the present invention, there is providedan electrochemical cell system comprising: (a) at least oneelectrochemical cell stack, each stack having a reactant fluid inlet;(b) a pressure transmitter arranged in the reactant fluid inlet of eachstack; and (c) a control unit for regulating the electrochemical cellsystem, the control unit receiving a signal value from the pressuretransmitter indicative of the reactant fluid pressure and comparing thesignal value with a stored value. The control unit may generate a leakindication signal if the signal value lower than the stored value. Thesignal value and the stored value may correspond with pressure in the atleast one electrochemical cell stack at a pre-set time period afterinitiating shut-down of the electrochemical cell system.

In accordance with a further aspect of the present invention, there isprovided an electrochemical cell system comprising at least oneelectrochemical cell stack, each stack having a reactant fluid inlet, apressure transmitter arranged in the reactant fluid inlet of each stack,and a control unit for regulating the electrochemical cell system, thecontrol unit: (i) receiving a plurality of signal values from thepressure transmitter indicative of the reactant fluid pressure; (ii)recording a time for the signal values to reach a pre-set pressurepoint; (iii) comparing the time to a pre-set threshold time; and (iv)generating a leak indication signal if the time is less than the pre-setthreshold time.

In accordance with a further aspect of the present invention, there isprovided a method of operating an electrochemical cell system, theelectrochemical cell system having at least one electrochemical cellstack and each stack having a reactant fluid inlet, the methodcomprising: (a) sensing a reactant fluid pressure in the reactant fluidinlet; (b) recording the reactant fluid pressure at a pre-set timeperiod from initiating shut-down of the electrochemical cell system; (c)comparing the reactant fluid pressure at the pre-set time period with astored pressure; and (d) indicating a leak if the reactant fluidpressure is lower than the stored pressure.

The method may further comprise: (a) comparing the reactant fluidpressure with a stored maximum pressure; and (b) indicating a pressurealarm signal if the reactant fluid pressure is higher than the storedmaximum pressure.

In accordance with a yet further aspect of the present invention, thereis provided a method of operating an electrochemical cell system, theelectrochemical cell system having at least one electrochemical cellstack and each stack having a reactant fluid inlet, the methodcomprising: (a) sensing a reactant fluid pressure in the reactant fluidinlet; (b) monitoring the reactant fluid pressure while initiatingshut-down of the electrochemical cell system; (c) recording a time forthe reactant fluid pressure to reach a pre-set pressure point; (d)comparing the time with a pre-set threshold time period; and (e)indicating a leak if the time is less than a pre-set threshold timeperiod.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention and to show moreclearly how it may be carried into effect, reference will now be made,by way of example, to the accompanying drawings which show, by way ofexample, one or more embodiments of the present invention and in which:

FIG. 1 a is a schematic view illustrating an electrochemical cell systemaccording to an embodiment of the present invention;

FIG. 1 b is a schematic view illustrating an electrochemical cell systemaccording to another embodiment of the present invention;

FIG. 2 a is a diagram illustrating the shut-down behavior of anelectrochemical cell system showing no stack leak; and

FIG. 2 b is a diagram illustrating the shut-down behavior of anelectrochemical cell system showing a stack leak is present.

DETAILED DESCRIPTION

Various apparatuses or methods will be described below to provide anexample of an embodiment of each claimed invention. No embodimentdescribed below limits any claimed invention and any claimed inventionmay cover apparatuses or methods that are not described below. Theclaimed inventions are not limited to apparatuses or methods having allof the features of any one apparatus or method described below or tofeatures common to multiple or all of the apparatuses described below.It is possible that an apparatus or method described below is not anembodiment of any claimed invention. The applicants, inventors andowners reserve all rights in any invention disclosed in an apparatus ormethod described below that is not claimed in this document and do notabandon, disclaim or dedicate to the public any such invention by itsdisclosure in this document.

Process fluid leaks in an electrochemical cell may still allow the cellstack to operate if the leaks are distributed across numerous cells. Theuser would possibly not notice any degradation in power output in thiscase. If the leaks are confined to one or a few cells, the stack willtypically have difficulty running at certain current densities and maycause a shut-down due to low cell voltage.

The present invention provides an apparatus and a method for providingdetection and indication of a leak in an electrochemical cell system bymonitoring the fuel fluid pressure in the anode inlet immediately afterand for a specific time after a shutdown of the system. The shutdown maybe a controlled shutdown or a shutdown generated by an emergency stopsituation (e-stop). A controlled shutdown takes place when anelectrochemical cell system is controlled to shut down normally and as aconsequence of the desired operating cycle without there being abnormalconditions detected in the system to cause the shutdown. An e-stopshutdown, conversely, takes place after abnormal conditions aredetected. In any case, as long as the ECU of the system has powerenabling it to perform leak detection, the leak test according to theinvention may be performed.

FIG. 1 a shows a fuel cell system 10 (which may be a fuel cell powermodule, FCPM) having a fuel cell stack 100.

It is to be appreciated that the fuel cell system according to thepresent invention can be of any configuration, there may be one or morefuel cell stacks and each fuel cell stack can comprise any number andtype of fuel cells, such as Proton Exchange Membrane (PEM) fuel cells,solid oxide fuel cells, alkaline fuel cells, etc. Examples of fuel cellsystem were disclosed U.S. Pat. Nos. 7,018,732 and 6,875,535, hereinincorporated in whole by reference.

The fuel cell stack 100 has an anode inlet 110, for fuel fluid, and acathode inlet 120, for oxidant. An anode outlet 130 expels depleted fueland a cathode outlet 140 expels superfluous oxidant.

It is not shown in the figures, but recirculation may be employed foreither the anode outlet (anode exhaust) and/or the cathode outlet(cathode exhaust). Typically, recirculation is employed to circulatefuel from the anode outlet to the anode inlet. Where recirculation isprovided, it is common to provide a purge valve for controlled andperiodic purging of a recirculation loop, for a variety of reason, e.g.,to prevent build up of contaminants in the recirculation gas. Further,the reactants (fuel and oxidant) may be humidified before entering thefuel cell stack, depending on the membrane material, and this is notshown in the figures either.

The fuel cell stack 100 has an electric output 150 connected to a load(not shown) and the voltage of the output is measured using a voltagesensor 160. Current drawn by the load and other characteristics can alsobe monitored.

A pressure transmitter 170 is arranged at the anode input 110 to measurethe fuel fluid pressure at the anode input 110. The pressure transmitter170 can be any suitable device or devices capable of monitoringpressure, and need not necessarily measure pressure constantly withtime. For example, a switch may be employed as the pressure transmitter170.

Similarly, a pressure transmitter 180 can be arranged at the cathodeinput 120 to measure the oxidant fluid pressure at the cathode input120.

A control unit 200 (or electronic control unit, ECU) is arranged toreceive the signal indicative of the fuel fluid pressure from thepressure transmitter 170 (or the oxidant fluid pressure from thepressure transmitter 180). The voltage sensor 160 communicates thevoltage signal to the control unit 200.

Hydrogenics Corporation, assignee of the present invention, typicallyemploys a FCPM system where the anode inlet pressure is biased from thecathode inlet pressure. This may be achieved by utilizing a forwardpressure regulator 210 (FPR) or similar device. The FPR controls theanode inlet pressure via a dome loading mechanism connected to thecathode inlet. For the cathode, in this embodiment, air is supplied by ablower as the oxidant, so that, at the cathode inlet to the stack 100,the pressure would be slightly in excess of atmospheric pressure. Itwill be understood that various oxidant sources can be used, e.g., pureoxygen can be used, and pressures higher or lower than atmosphericpressure can be maintained. The leak detection and indicationarrangement according to the present invention is not limited to apressure biased system, however, as there could be no bias at all.Further, a fuel inlet valve 220 (e.g., a hydrogen supply solenoid valve)and a fuel exhaust valve 230 (e.g., a hydrogen purge solenoid valve) maybe utilized in the system to close fluid communication of fuel to andfrom the stack. The fuel inlet valve and the fuel exhaust valve areadvantageously of the normally closed type.

At shut down, the fuel inlet and outlet valves 220, 230 are closed. Thecathode side of the stack 100, in this embodiment, remains open toatmospheric pressure. A fuel reservoir 190 can be provided to enablepassive blanketing of the electrodes, as disclosed in U.S. patentapplication Ser. No. 10/875,288, herein incorporated in whole byreference.

FIG. 1 b shows another embodiment of a fuel cell system 10, with thesame reference numbers used as for FIG. 1 a to describe correspondingfeatures. In this case, both the cathode inlet 120 and the cathodeoutlet 140 can be closed by oxidant inlet valve 240 and oxidant exhaustvalve 260, respectively.

It should be understood that fuel reservoir 190 is optional and need nottake the form of a separate tank(s) or vessels: the function of areservoir can be provided by piping alone, depending on the amount offuel necessary to passively blanket electrodes of the stack 100.

Referring to FIGS. 1 a and 1 b, all devices located between the valves220 and 230 can be included in the leak test. For instance, heatexchangers or similar devices and valves of different types, includingthe valves 220 and 230 themselves.

FIG. 2 a shows a diagram of the behavior of a cell stack according tothe embodiment shown in FIG. 1 a without any detectable leaks. The stackis shut down from a normal operation state at time zero (point O) butthe cells continue to produce electricity for a short period because ofresidual fuel and oxidant in the cells, thus a current is flowingthrough a discharge resistor (not shown) attached across the outputterminals of the stack, or dissipated through internal resistance of thestack. As the stack discharges and the hydrogen is electrochemicallyconsumed, the hydrogen pressure (dashed line) decreases. Once thehydrogen pressure reaches a certain level decided by the FPR setting(the slightly sloping plateau marked A, the plateau depends on thespring setting in the FPR), the FPR opens to allow fresh hydrogen toenter from the fuel reservoir 190 (shown in FIG. 1 a). The FPR maintainsthe anode pressure slightly above the cathode pressure until thehydrogen in the fuel reservoir 190 has been consumed. At point B, thehydrogen pressure reaches atmospheric pressure. After the hydrogen inthe anode side has been consumed at pressures above ambient, furtherhydrogen consumption by reaction drives the pressure at the anode inletmore and more negative until an equilibrium pressure point is reached(point C). Once the system has reached this maximum negative pressure,the anode inlet pressure starts to increase back to atmosphericconditions due mainly to crossover from the cathode to the anode side(the crossover fluid being mainly nitrogen). Regarding the stackvoltage, after a certain time, the reactants are depleted to cause thestack voltage (dotted line) to rapidly decay down to zero as the cellsare discharged across the discharge resistor. Atmospheric pressure isindicated with a dash/double-dotted line marked D. In one experiment,for example, it took about 25 minutes for the hydrogen pressure to reachapproximate atmospheric pressure, although this is not shown in FIG. 2a. The extent to which the anode inlet pressure drops below atmosphericpressure is dictated by the size of the fuel reservoir 190.

FIG. 2 b shows a diagram of the behavior of a cell stack with detectableleaks. The same reference letters have been used as for FIG. 2 a todescribe corresponding features. The voltage behavior is similar to thatshown in FIG. 2 a, but the hydrogen pressure reaches approximateatmospheric pressure in a much shorter time, in an experiment it tookabout 5 minutes. Atmospheric pressure is indicated with adash/double-dotted line.

Thus, cell stack leaks may be detected by detecting how quickly orslowly the hydrogen pressure in the anode inlet decays down toapproximately atmospheric pressure after a controlled shutdown or ane-stop of the electrochemical cell system. Leaks can be detected bycomparing signal values versus a stored “standard” value for the cellstack. Leaks can be detected vis-á-vis stored values either by (a)comparing the signal pressure at a pre-set time period after shut-downwith a stored value, or (b) comparing a recorded time for how long ittakes for the signal pressure to fall to a pre-set pressure point with apre-set threshold time period from shutdown. The pre-set pressure pointcan be, for example, the equilibrium pressure point for the system.Because of this, it should be appreciated that the control unit 200 canreceive the signal indicative of the reactant fluid pressure from thepressure transmitter 170/180 on a substantially continuous basis, or asa single reading taken, for example, after 5 minutes.

The control unit 200 compares the received signal value(s) with storedvalue(s) and generates a leak signal if the rate of decay of theelectrochemical cell system indicates a leak. The leak indication signalmay be a computer signal, a visual or audio signal or a combination, forexample a “check engine” light.

Additionally, the actual leak rate may be determined from the slope ofthe pressure decay. The control unit 200 would, for instance, comparestored actual pressure decay values obtained during a shutdown withstored pre-set values indicative of a no-leak cell stack shutdownpressure decay and with stored pre-set values indicative of a leaky cellstack shutdown pressure decay.

The sensed pressure is compared to either the stored pressure at apre-set time period after initiating shut-down, which is the ambientpressure obtained by the control unit 200, or, if the FPR has failed, astored value indicative of a maximum permissible anode inlet pressurefor the particular fuel cell stack type and construction. Thus, if thesensed fuel pressure (after the pre-set time period from shut-down) islower than the stored value, there is a system leak, if the sensed fuelpressure is greater than the stored value, there is no (measurable) leakand if the pressure is greater than the stored pre-set value (maximum“allowable” pressure), the FPR is likely malfunctioning.

A method of operating an electrochemical cell system according to thepresent invention has the steps of: (a) sensing a reactant fluidpressure in a reactant inlet to an electrochemical cell stack; (b)comparing the sensed reactant fluid pressure at a pre-set time periodfrom initiating shut-down of the electrochemical cell system with apre-set stored pressure; and (c) indicating a leak if the sensedreactant fluid pressure is lower than the stored pressure.

Another method of operating an electrochemical cell system according tothe present invention has the steps of: (a) sensing a reactant fluidpressure in a reactant inlet to an electrochemical cell stack; (b)monitoring the reactant fluid pressure and recording the time for thereactant fluid pressure to reach a pre-set pressure point; (c) comparingthe recorded time for the reactant fluid pressure with a pre-setthreshold time; and (d) indicating a leak if the recorded time is lessthan the pre-set time threshold.

It should be appreciated that the spirit of the present invention isconcerned with providing leak detection and indication for anelectrochemical cell system. The type and internal structure of theelectrochemical cell stack does not affect the design of the presentinvention. In other words, the present invention is applicable tovarious types of fuel cells, electrolyzers or other electrochemical cellsystems. The position, number, size and pattern of the electrochemicalcell stacks and peripheral devices are not necessarily identical asdisclosed herein.

It is anticipated that those having ordinary skill in this art can makevarious modification to the embodiment disclosed herein after learningthe teaching of the present invention. However, these modificationsshould be considered to fall under the protection scope of the inventionas defined in the following claims.

The invention claimed is:
 1. An electrochemical cell system comprising:a) at least one electrochemical cell stack, each stack having a reactantfluid inlet; b) a pressure transmitter arranged in the reactant fluidinlet of each stack; and c) a control unit for regulating theelectrochemical cell system, the control unit receiving a signal valuefrom the pressure transmitter indicative of the reactant fluid pressureand comparing the signal value with a stored value, wherein the signalvalue and the stored value correspond with pressure in the at least oneelectrochemical cell stack at a pre-set time period after initiatingshut-down of the electrochemical cell system.
 2. The electrochemicalcell system of claim 1, wherein the control unit generates a leakindication signal if the signal value is lower than the stored value. 3.The electrochemical cell system as recited in claim 2, wherein thecontrol unit generates a maximum pressure signal when the control unitcompares the received signal value with a stored maximum valueindicative of a maximum permissible reactant inlet pressure and thereceived signal value is higher than the stored maximum value.
 4. Theelectrochemical cell system of claim 3, wherein the reactant fluid isfuel fluid and the reactant fluid inlet is an anode inlet of the atleast one electrochemical cell stack.
 5. The electrochemical cell systemof claim 4, wherein the electrochemical cell stack is a proton exchangemembrane fuel cell stack.
 6. An electrochemical cell system comprising:a) at least one electrochemical cell stack, each stack having a reactantfluid inlet; b) a pressure transmitter arranged in the reactant fluidinlet of each stack; and c) a control unit for regulating theelectrochemical cell system, the control unit: i) receiving a pluralityof signal values from the pressure transmitter indicative of thereactant fluid pressure; ii) recording a time for the signal values toreach a pre-set pressure point; iii) comparing the time to a pre-setthreshold time; and iv) generating a leak indication signal if the timeis less than the pre-set threshold time.
 7. The electrochemical cellsystem of claim 6, wherein the pre-set pressure point corresponds withan equilibrium pressure point for the at least one electrochemical cellstack.
 8. The electrochemical cell system of claim 7, wherein thereactant fluid is fuel fluid and the reactant fluid inlet is an anodeinlet of the at least one electrochemical cell stack.
 9. Theelectrochemical cell system of claim 8, wherein the electrochemical cellstack is a proton exchange membrane fuel cell stack.
 10. A method ofoperating an electrochemical cell system, the electrochemical cellsystem having at least one electrochemical cell stack and each stackhaving a reactant fluid inlet, the method comprising: a) sensing areactant fluid pressure in the reactant fluid inlet; b) recording thereactant fluid pressure at a pre-set time period after initiatingshut-down of the electrochemical cell system; c) comparing the reactantfluid pressure at the pre-set time period with a stored pressurecorresponding with the pre-set time period after initiating shut-down ofthe electrochemical cell system; and d) indicating a leak if thereactant fluid pressure is lower than the stored pressure.
 11. Themethod of claim 10, wherein the method further comprises the steps of:a) comparing the reactant fluid pressure with a stored maximum pressure;and b) indicating a pressure alarm signal if the reactant fluid pressureis higher than the stored maximum pressure.
 12. A method of operating anelectrochemical cell system, the electrochemical cell system having atleast one electrochemical cell stack and each stack having a reactantfluid inlet, the method comprising: a) sensing a reactant fluid pressurein the reactant fluid inlet; b) monitoring the reactant fluid pressurewhile initiating shut-down of the electrochemical cell system; c)recording a time for the reactant fluid pressure to reach a pre-setpressure point; d) comparing the time with a pre-set threshold timeperiod; and e) indicating a leak if the time is less than a pre-setthreshold time period.